1. Field of the Invention
The present invention concerns the field of digital information transmission between a managing center and a plurality of user units connected to said center.
2. Technical Background
Such a situation arises for example in the field of digital pay television where a large number of subscribers' decoders are connected to a managing center. The information transmitted from the center comprises mainly data for updating rights contained in the security module associated to the decoder. These rights, bought by the user allowing the decryption of encrypted audio/video programs, have to be renewed frequently particularly when Video On Demand (VOD) services are used. The rights updating implies a generally short time connection of a decoder to the managing center. When the number of decoders reaches several millions and when they all attempt to connect in a same time interval, for example at subscriptions expiry, the managing center becomes rapidly overloaded.
For example in the field of wireless networks, a solution consists of calculating an access time at each connection attempt so that each unit can access the network in a given period. The document U.S. Pat. No. 6,490,645 describes a method wherein a plurality of users share an access to a resource such as a communication channel. Each user benefits from a priority and from a non-uniform probability distribution corresponding to this priority; the sum of the probability distributions being uniform. When a user desires to access to the resource, he selects a random number according his non-uniform probability distribution and calculates an access time based on this random number.
The document EP1346522 describes a solution consisting of connecting a first equipment seeking to establish a connection with a second one being in a reception state and to alternate the roles of emitter and receiver of each equipment. This alternating is carried out according to a probability distribution until the expiry of a predetermined test period or until the connection is established between the equipments. The length of the period during which an equipment plays the role of an emitter or of a receiver is controlled by the probability distribution.
The document WO2004/100590 describes a mobile communication system determining on the base of a number of mobile equipments the establishment of a communication either of a point-multi-points type or of a point-to-point type for transmitting multimedia data. Certain equipments are in standby mode while others maintain an active connection controlled by the system. The latter broadcasts a control message comprising an access probability factor in order to prevent the system overloading with responses to the control message. The connected equipments ignore the message and those in standby mode respond in function of the access probability factor. The system compares the number of received responses with a threshold and determines the type of connection to establish, i.e. point-multi-points or point-to-point. The system can also adapt the access probability factor in function of the number of responses.
The above described solutions are adapted to communication networks wherein the number of users desiring to access to a service provider is limited to several tens or hundreds. However, when this number reaches the millions, these solutions do not allow to solve, in a satisfactory way, the problem of the overloading of the managing center or of the central server. Moreover, certain units belonging to the network can be out of service at the moment they should have to be connected for their updating for example.